The acting national commander of the Comancheros and two members of the Rebels motorcycle gangs pleaded guilty to methamphetamine supply charges today as one of the nation's largest courtrooms was filled nearly to capacity in advance of a major drug trial.

Fourteen co-defendants had been set to face a jury on various charges related to Operation Cincinnati, which resulted in multiple raids and arrests in late 2020.

The dock in the High Court at Auckland was near overflowing with defendants and security as lawyers set the groundwork for the eight-week trial, which was scheduled to get under way today.

But by mid-morning, 10 of the defendants had resolved their cases by entering guilty pleas - leaving the courtroom one by one as Justice Neil Campbell entered their convictions and ordered them back for sentencing at a later date.

Two others entered guilty pleas to some but not all of the charges against them and will go to trial this week alongside two others set to plead not guilty to all charges.

Police made multiple arrests between October and December 2020 following a nine-month investigation by the National Organised Crime Group and Customs. Among those arrested were senior leaders of the Comancheros and Rebels gangs, who were alleged to have formed a partnership to manufacture or distribute methamphetamine and MDMA in Auckland and Christchurch, police said at the time.

While executing multiple search warrants, police recovered drugs, guns, ammunition and a large quantity of cash, authorities have previously said.

Police made multiple arrests and seized drugs and cash as part of Operation Cincinnati in late 2020. Photo / Supplied

Among those arrested was Seiana Fakaosilea, the Comancheros' acting national commander, who pleaded guilty today to two counts of supplying methamphetamine. Justice Campbell did not immediately enter convictions, however, because he will be pleading not guilty to other charges.

He is expected to stand trial beside co-defendants Taniela Mafileo, Jie Huang and Richard Pelikani.

With today's guilty pleas, the trial is now estimated to take three to four weeks. Jurors were instructed to return to the courthouse on Thursday for opening statements.

Those resolving their cases with guilty pleas today included two Rebels gang members appearing via audio-video link from Christchurch: Luke Mathers, who admitted to possession of methamphetamine for supply, possession of ammunition and possession of a firearm and Jarrod James Ronald Singer, who admitted to being an accessory after the fact to possession of methamphetamine for supply by flushing the drug down the toilet.

Those resolving their cases in the Auckland courtroom included Lemeki Namoa, who pleaded guilty to possession of methamphetamine and possession of the drug for supply; Samuel Tu'afoa Halaholo Jr, who pleaded guilty to possession of MDMA and methamphetamine for supply; Diamond Shaquille Katoa, who pleaded guilty to possession of methamphetamine for supply; Rhakim Eneliko Mataia, who pleaded guilty to possession of methamphetamine for supply, unlawful possession of a firearm and unlawful possession of ammunition; Joshua Rapi Tofa-Tulisi, who pleaded guilty to possession of MDMA for supply; Elizabeth Maria Bolea, who pleaded guilty to participating in an organised criminal group to possession and supply methamphetamine; Siolei Fakana'ana'a Fangupo Fakafanua, who pleaded guilty to possession of ammunition and conspiracy to commit aggravated robbery; and a man listed in court documents as "W" due to continuing interim name suppression who pleaded guilty to conspiracy to commit aggravated robbery.

Crown prosecutor Robin McCoubrey agreed to withdraw additional charges from most of the defendants who pleaded guilty.

Justice Campbell set a nominal sentencing date for each of the defendants for September. New dates are likely to be set after the trial concludes and after lawyers confer about which co-defendants it would be most logical to group together for the hearings, the judge said.

Many of those who pleaded guilty today indicated they may seek home detention or community supervision instead of prison terms.